1. Field of the Invention
The present invention relates to an image forming apparatus capable of switching the beam pitch of optical beams that are emitted from a multi-beam emitting source by deflecting the optical beams with a liquid crystal deflecting element, and a method for controlling the image forming apparatus.
2. Description of the Related Art
Conventionally, there is known a method for adjusting the beam position on a surface to be scanned in such a way that a liquid crystal element, which is driven by the application of a voltage, is provided in a light source unit or just downstream of the light source unit in an optical scanning device used in an optical writing system of an electrophotographic image forming apparatus, where an optical beam is deflected by the liquid crystal element depending on a drive voltage (see Japanese Patent No. 4197431). Hereinafter, such a liquid crystal element used for deflecting an optical beam is referred to as a “liquid crystal deflecting element”.
FIGS. 22A and 22B conceptually illustrate how optical beams emitted from a multi-beam emitting source are deflected by a liquid crystal deflecting element according to a conventional technology. FIGS. 22A and 22B illustrate examples of 2-beam scanning and 4-beam scanning, respectively. The multi-beam emitting source is composed of, for example, a laser diode (LD) or an LD array.
FIG. 22A illustrates an example where two beams fixed at a pitch of 21.6 micrometers (equivalent to a scan density of 1200 dots per inch (dpi)), which are emitted from a multi-beam emitting source, are deflected by passing through a liquid crystal deflecting element, thereby being changed of the pitch of the beams on a surface to be scanned in a range from 10.6 micrometers (equivalent to a scan density of 2400 dpi) to 42.3 micrometers (equivalent to a scan density of 600 dpi). The pitch can be decreased by deflecting the two beams inward; the pitch can be increased by deflecting the two beams outward.
FIG. 22B illustrates an example where four beams fixed at a pitch equivalent to a scan density of 600 dpi pass through liquid crystal deflecting elements, thereby changing the pitch of the beams on a surface to be scanned to a pitch of 21.2 micrometers (equivalent to a scan density of 1200 dpi). In this case, to irradiate the surface to be scanned with the four beams at regular intervals, the deflection angle differs between the two outer beams and the two inner beams.
In this manner, in the case of changing the scan density, a drive voltage depending on an amount of deflection is applied to the liquid crystal deflecting element acting on the beams, whereby the surface to be scanned can be scanned at the desired scan density.
In the method according to the conventional technology described above with reference to FIGS. 22A and 22B, in any of these cases of 2-beam scanning and 4-beam scanning, to deflect beams independently of one another, it is necessary to provide N number of liquid crystal deflecting elements with respect to N number of beams. Or, also in a case of using a single liquid crystal deflecting element capable of controlling the deflection for each effective area, it is necessary to provide a liquid crystal deflecting element having N number of effective areas.
Furthermore, there may be a case where one out of N number of beams is fixed and N−1 number of the other beams are deflected. However, also in this case, N−1 number of liquid crystal deflecting elements or effective areas are required. In either case, the number of required liquid crystal deflecting elements (or effective areas) is increased with an increase in the number of beams.
Moreover, since the deflection angle differs from one beam to another, a different drive voltage needs to be set for each liquid crystal deflecting element or each effective area, which makes the control complicated.